The present invention relates to a directional coupler that can be used in a polarization-independent manner and to a design method thereof, an optical waveguide element and a wavelength filter.
In an information processing device that requires high speed signal processing, electrical wiring bandwidth limitations cause bottlenecks. As a result, with an increase in the amount of information transmission, optical cable technology is attracting attention. In optical cable technology, optical devices are used that use optical fiber and optical waveguide elements as a transmission medium, and the transmission of information between elements, between boards and between chips, inside the information processing device is performed using optical signals.
An optical device is configured to be provided with optical elements, such as an optical transmitter, an optical receiver and the like. These optical elements can be spatially coupled together using a lens, for example, after performing complex optical axis alignment in order to align central positions (a light receiving position or a light emission position) of each of the optical elements with design positions.
Here, as means to couple each of the optical elements, technology exists in which an optical waveguide element is used in place of the lens (refer to Japanese Laid-Open Patent Publication No. 2011-77133, for example). When using the optical waveguide element, light is confined and propagated inside an optical waveguide and thus, in contrast to the case in which the lens is used, it is not necessary to perform the complex optical axis alignment. As a result, an assembly process of the optical device becomes simple and is suitable for mass production.
In particular, in an optical waveguide element that uses silicon (Si), which is the same material as that of a substrate of an electronic function circuit, as a waveguide material, it is anticipated that photoelectric fusion (silicon photonics), in which the electronic function circuit and an optical function circuit are collectively formed, will be realized. (refer to IEEE Journal of Selected Topics in Quantum Electronics, vol. 11, 2005, p.232-240 and IEEE Journal of Selected Topics in Quantum Electronics, vol. 12, No. 6, November/December 2006, p.1371-1379, for example). In silicon photonics, at a time of manufacture, it is possible to use semiconductor manufacturing technology, which is a mature technology.
As a structure of the optical waveguide element, there is an Si wire waveguide, for example. In the Si wire waveguide, an optical waveguide core that is the actual transmission path of the light is formed using Si as a material. Then, a periphery of the optical waveguide core is covered with a cladding made of silicon oxide (SiO2) or the like, for example, which has a lower refractive index than Si. With this type of structure, as there is an extremely large difference in the refractive index between the optical waveguide core and the cladding, it is possible to strongly confine the light inside the optical waveguide core. As a result, it is possible to realize a compact curved waveguide in which a radius of curvature is around several nm, for example. Thus, it is possible to create an optical circuit having a similar size to the electronic circuit, which is advantageous in making the optical device as a whole more compact.
Further, if a silicon on insulator (SOI) substrate is used for the Si wire waveguide, it is possible to easily manufacture. First, the SOI substrate is prepared which is formed by sequentially laminating a support substrate layer, an SiO2 layer and an Si layer (SOI layer). Next, the optical waveguide core is formed by patterning the SOI substrate. Then, the optical waveguide core is covered by an SiO2 layer. In this manner, the Si wire waveguide can be obtained in which the optical waveguide core is covered by the SiO2 layer as a cladding.
When the optical device is used in a communication system that uses wavelength multiplexing technology, such as a passive optical network (PON) etc., it is necessary to have an element that switches a path of an optical signal for each wavelength. In order to realize this, there is a structure that uses an optical waveguide element which has been imparted a function as a wavelength filter.
The wavelength filter that uses the optical waveguide element is, for example, a Mach-Zehnder wavelength filter. As a coupler of the Mach-Zehnder wavelength filter, a directional coupler that is formed of the Si wire waveguide is used.
Here, characteristics of the Si wire waveguide differ depending on the polarization. Therefore, a disadvantage of the directional coupler using the Si wire waveguide is that polarization dependence occurs. As a method to resolve the polarization dependence, it is conceivable to adjust a separation distance between a pair of the optical waveguide cores that form the directional coupler, along with adjusting a thickness and a width of the optical waveguide cores.